The invention relates to a device for encapsulating electronic components, mounted on so-called lead frames, in a mould assembled from two mould halves movable relative to each other and closable onto each other, comprising:
means for causing the mould halves to move and to close, and PA1 means for exerting pressure on encapsulating material which can be placed in cavities of the mould. PA1 a) placing the mould halves into mutual contact by causing a one mould half to move toward a second mould half, PA1 b) carrying along of the second mould half by the first mould half over a determined distance, wherein a force directed to the first mould half is exerted on the second mould half, PA1 c) building up a closing force between the two mould halves after reaching a fixed position of the second mould half.
The invention also relates to methods for operating such a device.
Existing presses of the type mentioned in the preamble operate at limited speeds and are susceptible to malfunction and require frequent maintenance.
The present invention has for its object to provide an improved device and improved methods for operating such a device with which electronic components can be encapsulated at higher speed and with less chance of breakdown.
The invention provides for this purpose a device of the type stated in the preamble wherein the means for causing the mould halves to move and to close are formed by a rotatable eccentric which can be coupled to one of the mould halves with interposing of at least one connecting rod. The dimensions of the eccentric are preferably such that the position of the eccentric in a fully opened position of the mould halves is rotated less than 180.degree. relative to the position of the eccentric in which the mould halves close onto each other. A first advantage of a rotatable eccentric for causing the mould halves to move and close is that the movement can take place relatively rapidly. A rotation of the eccentric of a maximum of 180.degree. can take place very rapidly, while an alternative such as for instance driving of a mould half with a screw spindle is less rapid. A second important advantage of the rotatable eccentric is the very favourable movement characteristic which can be derived therefrom. When reaching the two extreme positions of the mould half to be driven, the position of the eccentric will, at a turn of 180.degree. of the eccentric as seen from the driven-out mould half, be situated almost completely behind respectively almost completely in front of the rotation axis of the eccentric. The speed of movement of the driven mould half will, at constant rotation speed of the eccentric, correspond with semi-sinusoids, wherein the speeds close to the end positions approach 0 metres per second. While this ideal situation is only achieved at a rotation of the eccentric through 180.degree., in practice this will have to be slightly less than 180.degree.. A very significant portion of the semi-sinusoids will however be recognizable in the movement characteristic of the driven mould half. This movement characteristic increases the drive speed of the eccentric still further since in the critical positions, i.e. when approaching the end positions of the driven mould half, the speed of the driven mould half will be lower than the speed of the driven mould half when it is situated between the two end positions. Another advantage of this movement characteristic is that the load of the machine parts as well as the energy consumption will be limited compared to existing encapsulating devices. Yet another advantage is that an encapsulating device with a rotatable eccentric requires relatively little maintenance.
In a preferred embodiment the device is provided with a force sensor for measuring the force with which the mould halves close, which force sensor is connected to the drive of the eccentric in order to supply a signal to the drive of the eccentric when a desired closing force has been reached. Due to this step the eccentric not only functions to cause displacement of the driven mould half but the desired closing force can also be provided using the eccentric.
The connecting rod for coupling the rotatable eccentric to one of the mould halves is preferably curved respectively angular. This shape makes it possible to apply a continuous rotation shaft for the eccentric. The curved respectively angular connecting rod in fact enables rotation of the eccentric to a position in which it lies on the side of the rotation axis remote from the mould half. Since the eccentric does not run through a full circular movement a crankshaft construction with interrupted rotation shaft can be avoided. This increases the robustness of the device.
In a preferred embodiment of the invention the rotatable eccentric can be coupled to one of the mould halves with interposing of the rod mechanisms, which rod mechanisms for obtaining a lever comprise at least one drive member which is linearly movable by the connecting rod, which drive member is coupled with at least two pivotable arms to at least two knee levers, which knee levers engage on one side at fixed positions and engage on the opposite side on a fixing part for a mould half. By means of this construction a precise linear translation of the fixing part is obtained. By means of the knee levers large force amplification also occurs at the end of the stroke of the encapsulating device. Using a relatively simple drive sufficient closing force can nevertheless be obtained hereby to encapsulate electronic components.
The invention also comprises a method for driving a device such as described in the preamble, wherein by causing an eccentric to rotate a mould half coupled to the eccentric is moved. The advantages of moving a mould half with an eccentric have already been elucidated above.
The invention also comprises a device of the type stated in the preamble wherein a one mould half is connectable to the means for causing the mould halves to move and close and the second mould half is connectable to a counter-plate which forms part of the device, which counter-plate exerts a force directed to the one mould half and which counter-plate is displaceable between two end positions such that it can be carried along over a distance by the one driven mould half when the mould halves make mutual contact. Using this construction it is possible to move the mould halves against each other at relatively great speed without this directly resulting in a large closing force. When the counter-plate is situated between the two end positions, the closing force between the mould halves is determined by the force which the counter-plate exerts in the direction of the one mould half. In this manner a fixed closing force can be realized wherein it is possible to detect whether the mould halves close properly onto each other. The advantage hereof is that damage to mould parts can be prevented or at least limited by not increasing the closing force further when the mould halves do not close onto each other correctly. This may for instance be the case when a lead frame is not placed correctly in a mould half or when a pellet of encapsulating material is positioned incorrectly. Yet another advantage is that the safety of such an encapsulating device is improved. If a body part becomes caught between the mould halves it is possible to react to this when the closing force is still limited, i.e. when the counter-plate is still situated between the two end positions.
The encapsulating device with the movable counter-plate preferably comprises a guide along which the counter-plate is displaceable and the end positions are preferably determined by stops. These steps provide the counter-plate with the desired freedom of movement and also make it possible to increase the closing force in one end position to the closing force required during encapsulating.
In preference the counter-plate is substantially vertically displaceable and the force directed to the one mould half then preferably consists at least of the gravitational force on the counter-plate. The substantially vertically displaceable counter-plate provides a force directed toward the one mould half without additional steps, since the gravitational force on the counter-plate is after all already present. If this force is not sufficiently large, it is possible to adapt the weight of the counter-plate. Another possibility for correcting a force which is respectively too large or too small is to arrange springs between a frame of the encapsulating device and the counter-plate. These springs can be either draw or pressure springs depending on the position where they engage on the frame. An additional force may for instance be required when the lead frames must be oriented by closing of the mould halves.
The encapsulating device is preferably provided with sensors for determining at least one end position of the counter-plate. The encapsulating device is preferably also provided with sensors for determining the distance between the mould halves. By means of the sensors it is possible to automate the control and monitoring of the operation of the encapsulating device.
The invention additionally comprises a method for closing a mould assembled from two mould halves movable relative to each other and closable onto each other for encapsulating electronic components mounted on so-called lead frames, wherein during closing of the mould the following steps are successively performed of:
Herein the distance between the mould parts is preferably measured during step b and step c is only initiated when the mould parts are connected. The advantages as already described for the device with the movable counter-plate also emerge in execution of this method. It is therefore also possible with this method to produce at greater speed, with less risk of damage to mould parts, a greater safety and a better controlled quality of the encapsulated product.
Another aspect of the encapsulating device referred to in the preamble comprises a counter-plate to which at least one of the mould halves is connectable, which counter-plate comprises: a plurality of stacked, substantially plate-like parts, between which parts at least one shaft is placed. This counter-plate preferably comprises two plate-like parts between which at least two parallel shafts are placed. Herein recesses are preferably arranged in the plate-like parts for co-action with the shafts. A significant drawback of the existing integrally formed counter-plates can be prevented by a counter-plate manufactured from a number of plate-like parts between which at least one shaft is placed. An existing counter-plate does have a tendency to deform slightly when a large closing force is built up. When a large closing force is being built up the counter-plate must be supported This can be achieved for instance by one or more stops against which the counter-plate then rests. At the positions where the counter-plate is not supported it subsequently tends to buckle up slightly. While in the case of a relatively thick counter-plate this deformation will be small it can still have very disturbing consequences for the encapsulating process since the closing force is not distributed uniformly over the mould halves. In the case of a lateral supporting of a counter-plate it is for instance possible for the closing force in the middle of a mould half to be considerably smaller than at the edges of this mould half. This is disadvantageous in the control of the quality of the encapsulated products.
By now dividing at least one counter-plate into layers between which at least one shaft is placed the counter-plate will no longer, or practically no longer, deform. This leads to a better controllable encapsulating result, while the wear of the mould halves will also take place evenly.
A simpler construction of the counter-plate with which at least a part of the above described effect can be obtained can consist of dividing the counter-plate into two substantially plate-like parts, wherein at least one of the two plates is provided on the contact side to the other plate with protruding parts in the form of for instance protrusions or ridges. The operation of the above described shafts is then taken over by these protruding parts. Although in respect of the deformation of the whole counter-plate this construction probably has less good properties than the counter-plate in which shafts are arranged between two plate-like parts this construction is still preferred to one solid counter-plate. An advantage of this simplified construction is that the construction height of the counter-plate with two plate-like parts, of which at least one is provided with overhanging portions, can be limited relative to the construction height wherein shafts must be arranged between the plate-like parts.
The invention also comprises a device of the type stated in the preamble wherein the means for exerting pressure on the encapsulating material comprise at least two screw spindles, which screw spindles are driven by a central drive and which, by rotation of the screw spindles, support a displaceable mounting point for at least one plunger. The screw spindles are preferably driven by a central motor with interposing of belts. Two screw spindles at a time preferably also support a beam which is displaceable by rotation of the screw spindles and on which at least one plunger is mountable. Instead of using a transfer plate on which the plungers engage, generally with interposing of a plunger block, a plunger displacement using screw spindles according to this invention has the advantage that it takes up very limited space. Another advantage of the centrally driven screw spindles is that this results in an exactly similar placing of the plungers, which in turn results in an improved quality of the encapsulated products.